Devices capable of switching between mirror-like and transparent states find many applications in architectural and transportation energy conservation, lighting and displays, aerospace insulation control, and optical communications systems. Such devices, termed xe2x80x9cswitchable mirrorsxe2x80x9d, based on rare earth hydrides were discovered by Huiberts et al., Nature 380, 231 (1996), who observed a reversible metal-to-insulator transition when a thin film (150 to 500 nm) of yttrium or lanthanum coated with a thin layer of palladium was exposed to hydrogen gas. The transition accompanies conversion of a metallic dihydride phase to a semiconducting trihydride. Rare earth-magnesium alloy films were subsequently found to be superior to the pure lanthanides in maximum transparency and mirror-state reflectivity, see Van der Sluis et al., Appl. Phys. Lett. 70, 3356 (1997).
Phase separation appears to occur when these alloys take up hydrogen, giving transparent MgH2 and LnH2-3, both of which may participate in the switching mechanism. Because the rare earths are highly vulnerable to oxidation, a Pd overlayer at least 5 nm thick is required for films exposed to air or to an alkaline electrolyte. Although the Pd catalyzes the uptake and removal of hydrogen, it limits the maximum transparency of the composite film to about 50%.
Other fields of art have developed materials that exhibit electrochromism. Among the many transition metals and alloys that have been investigated for use in low pressure hydrogen storage devices or as electrodes in secondary batteries, a few are known to form semiconducting hydride phases such as Mg2NiH4, Mg2CoH5, and Mg2FeH6, see Reilly et al., Inorg. Chem. 7, 2254, (1968); Zolliker et al., Inorg. Chem., 24, 4177, (1985) and Didisheim et al., Inorg. Chem., 23, 1953, (1984).
Electrochromism is a term used to describe a reversible change of color exhibited by some materials when placed in an electric field. Some materials can exhibit a mirror-like state when no electric field is present, and change to a transparent state when an electric field is applied, and vice versa. By laminating thin films of electrochromic materials between electrodes, an electrochromic device is created, also termed a xe2x80x9cswitchable mirrorxe2x80x9d or xe2x80x9cswitching devicexe2x80x9d. Switchable mirrors may also be created by laminating the materials disclosed herein and applying a hydrogen source to the film. Upon the application of hydrogen the materials exhibit a change to a colored/transparent state. Removing the hydrogen cause the materials to switch back to their mirror-like state.
A new class of switchable mirrors based on novel electrochromic materials has been discovered. The rapid, reversible conversion from the highly reflecting metallic state to a transparent semiconducting state can be produced by either electrochromic or gasochromic means.
This invention contemplates transition metal compositions that are useful as electrochromic materials. Generally, these transition metal compositions are transition metal compositions formed with magnesium, i.e. which include magnesium. These compositions may be metals, alloys, hydrides or mixtures of alloys, metals and/or hydrides. Further contemplated by this invention are ternary combinations of two transition metals and magnesium. Examples are: NixCo1-xMgy (with 0 less than x less than 1, and 2 less than y less than 12, preferably 3 less than y less than 8), or others containing first row transition metal elements Zn, Cu, Ni, Go, Fe, Mn, Cr, V, Ti, Sc. Further examples of ternary combinations of two transition metals and magnesium include transition metal alloys FeTi, VTi, NiTi2, NiZr2, Nb and Ta alloys; also contemplated are hydrides containing Ca, Sr, Ba, Li, Na, Mg, K or Rb with any second or third row transition metal. The latter are known to undergo metal to insulator transitions when the hydrides are formed from the metal alloys or mixtures. Among the materials contemplated for the instant invention are those known to form semiconducting hydride phases such as transition metal alloys FeTi, VTi, NiTi2, NiZr2, Nb and Ta alloys.
The electrochromic materials disclosed in this invention are more resistant to oxidation and are more highly reflecting in the metallic state than are the rare earth compounds of previous electrochromic materials.